Polishing slurries currently used for planarization in the semiconductor industry are typically aqueous suspensions comprised of an abrasive (such as alumina), an organic dispersant, and a suitable oxidizing agent. This process is known as chemical-mechanical polishing (CMP). The oxidizing agent works to convert the metal surface to a more easily polishable metal oxide. Some oxidizing agents employed in commercially-available or proprietary slurries are metal compounds that are present in significant concentrations. Other chemicals added to slurries to improve dispersion or otherwise enhance performance often are metal-based. Sodium, potassium, and iron compounds are frequently used in slurry formulations, and significant amounts of these metal impurities can remain on the wafer after polishing and normal cleaning.
Currently, a typical cleaning process performed on wafers after metal plug polish processing is a 2-step, double-sided brush scrubbing with 2% NH.sub.4 OH. This is a reasonably effective cleaning process for particulate matter from the slurry, and it does significantly reduces the ionic and organic contaminants adhering to the wafer. However, surface-sensitive TXRF (Total-internal-reflectance X-Ray Fluorescence) analysis of wafers polished with an iron-containing slurry, and scrubbed with 2% NH.sub.4 OH, indicate that very high levels of Fe and Fe containing contaminants--ranging from 1.2-2.4 E14 atoms/cm.sup.2 --are bound at the wafer surface. This is roughly equivalent to 0.1 monolayer of Fe. In this instance other metallic impurities, notably K, Na, Ti, and Al, are detected at much lower levels (E11-E12 atoms/cm.sup.2) by TXRF and SIMS.
After CMP, although generally the Fe contaminant is present at a layer in the device that is somewhat removed from the transistors at the Si surface, since iron is known to degrade gate oxides and to impair device characteristics, prudent steps should be taken to remove the Fe from the wafer after polishing. Consequently, them is a need to develop an additional (or alternate) cleaning process to remove Fe and other impurities from post CMP wafers.